1. Field of the Invention
The present invention relates to a control device and a control method for an internal combustion engine, and more particularly, to a control device for an internal combustion engine in which a supercharger is mounted, for example.
2. Description of the Related Art
There has been known a turbocharger having a supercharger that operates by rotating a turbine by using an exhaust gas, which is mounted in an intake passage of an internal combustion engine (hereinafter referred to simply as “engine”) for the purpose of improvement of an output of the engine. In the following, an operation range in which supercharging is performed by the supercharger is referred to as a supercharging operation range, whereas an operation range in which the supercharging is not performed by the supercharger is referred to as a non-supercharging operation range.
In a state in which the engine rotates at a high speed under a high load, The turbocharger has a fear in that a supercharging pressure is increased to become larger than needed with the result that the engine is broken. Therefore, in general, an exhaust bypass passage is provided upstream of the turbine. In the exhaust bypass passage, a wastegate valve (hereinafter referred to as “WGV”) is provided. The WGV diverts a part of the exhaust gas flowing through an exhaust passage to the exhaust bypass passage so as to adjust an inflow of the exhaust gas into the turbine. In this manner, a pressure (supercharging pressure) in an intake passage of the engine is controlled to an appropriate level.
In general, the WGV is driven by using a positive-pressure actuator. The mechanism of driving the WGV is as follows. Specifically, the intake passage of the engine (particularly, an upstream portion of a throttle valve, in which the pressure increases) and a wastegate actuator (hereinafter referred to as “WGA”) are connected to each other. When the pressure in the intake passage of the engine becomes higher than an atmospheric pressure, an operation of the WGA is enabled. In general, the WGV, which is a bypass valve, is maintained in a fully-closed position until the operation of the WGA is enabled.
The turbine provided in the exhaust passage blocks the passage of the exhaust gas from a combustion chamber, and therefore becomes a factor for an increase in exhaust pressure. As the exhaust pressure increases, an exhaust loss becomes larger to lower the efficiency of the engine.
With a conventional configuration, an opening degree of the WGV cannot be operated unless the pressure in the intake passage of the engine increases. Therefore, when the pressure in the intake passage such as that during the operation of the engine in the non-supercharging operation range is equal to or lower than a predetermined value, the WGV is not operated even when an opening-degree changing request for the WGV is issued. Therefore, the efficiency of the engine is lowered.
In view of the above-mentioned situation, in recent years, there has been proposed a system including a motorized WGA, which is capable of operating the WGV when needed regardless of the pressure in the intake passage (for example, see Japanese Patent Application Laid-open No. 2006-274831).
The exhaust loss can be reduced by opening the WGV during the operation of the engine in the non-supercharging operation range. As a result, the engine efficiency is enhanced to improve fuel consumption. However, a turbine rpm is lowered with the reduction in exhaust pressure. Therefore, when an acceleration request is subsequently issued, turbo lag becomes longer. As a result, there arises a problem in that acceleration performance of the engine is degraded.
Thus, according to one of the conventional technologies, the opening degree of the WGV is set to about 20% (WGV opening-degree: full closure=0% and full opening=100%) when the engine operates in the non-supercharging operation range. By fully closing the WGV in response to the acceleration request using an accelerator signal, a supercharging-pressure response (acceleration responsiveness) at the time of acceleration is ensured while the fuel consumption is improved.
However, the method described in Japanese Patent Application Laid-open No. 2006-274831 has the following problem. In a low engine-load range included in the non-supercharging operation range, the acceleration responsiveness at the time of acceleration is degraded by opening the WGV by 20%.